1. Field of the Invention
The present invention generally relates to an optical semiconductor device and a method of fabricating the same, and more particularly, to the technique of forming a passivation film on an end surface of a semiconductor laser diode.
2. Description of the Related Art
Recently, many types of high-power semiconductor laser diodes have been proposed. Powerful laser diodes have the problem of catastrophic optical damage (hereinafter simply referred to COD). Power up of the semiconductor laser diodes will not be realized unless a high COD level can be achieved. COD relates to the state of the end surface (facet) of the laser diode. If there is an impurity (for example, natural oxide film) or lattice defect exists on the end surface, light will be absorbed at such defective portions, this absorption resulting in many electrons and holes. Then, the electrons and holes are recombined, which generates heat. Heat thus generated will damage the end surface of the laser diode.
As means for improving the COD level, a proposal of forming a passivation film on the end surface has been known (for example, Japanese Unexamined Patent Publication Nos. 7-176819 and 9-64453). As another practical proposal, it is known to grow a passivation film of alumina (Al2O3) or silicon oxide (SiO2) by electron beam deposition.
However, there is a limit on improvements of the COD level by alumina and silicon oxide. In order to further improve the COD level, an aluminum nitride (AlN) film may be essential. In order to grow AlN, aluminum and nitrogen are separately supplied in the chamber, then reacting and growing on a laminated layer structure. In growth, nitrogen gas and ions enter into the laminated layer structure containing aluminum via its end surface (facet). The components thus entering into the laminated layer structure roughens the end surface thereof that is originally flat. The rough end surface serves as a light absorption layer and degrades the COD level.
The present invention has been made in view of the above and provides an optical laser diode and a method of fabricating the same.
A more specific object of the present invention is to effectively prevent components in the passivation film from roughening an end surface of a laminated layer structure of the optical semiconductor device and to thus improve COD.
According to an aspect of the present invention, there is provided an optical semiconductor device comprising: a laminated layer structure; an intermediate film formed on an end surface of the laminated layer structure; and a passivation film formed on the intermediate film, the passivation film having a quantity of ion projection larger than that of the intermediate film.
According to another aspect of the present invention, there is provided a method of fabricating an optical semiconductor device comprising the steps of: (a) forming an intermediate film on an end surface of a laminated layer structure of the optical semiconductor device at a first rate of growth by ion-assisted deposition; and (b) forming a passivation film on the intermediate film at a second rate of growth lower than the first rate of growth by ion-assisted deposition.
According to yet another aspect of the present invention, there is provided a method of fabricating an optical semiconductor device comprising the steps of: (a) forming an intermediate film on an end surface of a laminated layer structure of the optical semiconductor device without ion projection; and (b) forming a passivation film on the intermediate film by ion-assisted deposition.
According to a further aspect of the present invention, there is provided a method of fabricating an optical semiconductor device comprising the steps of: (a) forming an intermediate film that does not contain nitrogen substantially on an end surface of a laminated layer structure of the optical semiconductor device; and (b) forming a passivation film on the intermediate film containing nitrogen.
According to a still further aspect of the present invention, there is provided a method of fabricating an optical semiconductor device comprising a step of growing aluminum nitride on an end surface of a laminated layer structure of the optical semiconductor device while projecting nitrogen ions onto the end surface.